Microsatellite instability (MSI) and deficient DNA mismatch repair (dMMR) colorectal cancers stand out among malignancies because they tend to generate immunogenic signals that immune cells can detect more readily. Clinically, these tumors often respond better to immune checkpoint inhibitors, yet the underlying biology has remained incompletely defined.
A new study from the Keck School of Medicine of USC reports a mechanism that links defective DNA repair to a self-amplifying immune feedback loop. Using a mouse model engineered to disrupt a key DNA repair gene in tumor cells, the researchers observed how immune recognition evolves as tumors die.
The team focused on two genes: Death Receptor 5 (DR5) and Ligase 3 (Lig3). In the MSI/dMMR context, tumor cells increased DR5 gene activity, initiating pathways that promote their own demise. As cancer cells progressed through this death program, they released extrachromosomal circular DNA (eccDNA), small circular DNA fragments separate from the chromosomes.
When Lig3 activity was high, eccDNA release increased, suggesting a quantitative connection between DNA repair dysfunction, eccDNA biogenesis, and immunogenic output. Those DNA fragments then amplified immune signaling beyond the initial tumor cell response, sustaining an ongoing cycle in which continued tumor cell death helps recruit and activate the immune system.
To test whether this mechanism translates to humans, the researchers analyzed gene expression patterns in colorectal tumor datasets. Tumors showing higher DR5 and Lig3 activity were more likely to respond to immune checkpoint blockade, aligning patient outcomes with what had been observed in mice.
The work was published in Gastroenterology and funded in part by the NIH. Importantly, it identifies a potential strategy for broadening the immunotherapy-responsive fraction of colorectal cancer by pharmacologically steering the DR5 pathway and/or enhancing eccDNA release.
Looking ahead, the authors aim to determine whether DR5/Lig3 activity can serve as a biomarker to personalize treatment decisions and to evaluate related MSI/dMMR cancers, including subsets of ovarian and endometrial tumors. More studies are needed before interventions can be tested in patients, particularly involving human colorectal tumor cells.
Subject of Research: Animals
Article Title: A DR5/Ligase 3-mediated feedback loop perpetuates immunogenicity in mismatch repair deficient colorectal cancer
News Publication Date: 16-Jul-2026
Web References: https://keck.usc.edu/
References: Gastroenterology; funded in part by the NIH (R01CA236271, R01CA247231, R01CA248112, R01CA215481, R01CA260900)
Image Credits: Not provided
Keywords: colorectal cancer; immunotherapy; DNA repair; translational medicine; cancer; MSI; dMMR; DR5; Ligase 3; eccDNA; immune checkpoint inhibitors
Tags: circular DNA fragments and immune signaling in colorectal cancerColorectal cancer immunotherapy responseDNA repair dysfunction and tumor immunogenicityeccDNA release and immune activation in cancergenetic factors influencing immunotherapy effectivenessimmune checkpoint inhibitors in colorectal cancermechanisms of immune feedback loops in MSI/dMMR tumorsmicrosatellite instability and DNA mismatch repair deficiencyrole of DR5 and Ligase 3 in tumor cell deathtumor cell death pathways and immune system recruitmentUSC Keck School of Medicine
